We present the first measurement of the rate of Type la supernovae at high redshift. The result is derived by using a large subset of data from the Supernova Cosmology Project. Three supernovae were discovered in a surveyed area of 1.7 deg2. The survey spanned a ́3 week baseline and used images with 3 σ limiting magnitudes of R ∼ 23. We present our methods for estimating the numbers of galaxies and the number of solar luminosities to which the survey is sensitive, as well as the supernova detection efficiency, which is used to determine the control time, the effective time for which the survey is sensitive to a Type la event. We derive a rest-frame Type la supernova (SN) rate at z ∼ 0.4 of 0.82-0.37-0.25+0.54+0.37 h2 SNu (1 SNu = 1 SN per century per 1010LḂ), where the first uncertainty is statistical and the second includes systematic effects. For the purposes of observers, we also determine the rate of SNe, per sky area surveyed, to be 34.4-16.2+23.9 SNe yr-1 deg-2 for SN magnitudes in the range 21.3 < R < 22.3.